For applications such as measurement of thin film thickness or index of refraction of semiconductor wafers with at least one deposited or grown thin film layer, it is desirable to measure near normal incidence reflection (using reflected broadband UV, visible and near infrared electromagnetic radiation) from a small region on the sample. Several methods currently exist for measuring small spot size near normal incidence spectroscopic reflection. However, there are drawbacks to each of these methods that are overcome by the present invention.
One method is to use a refractive microscope objective to focus electromagnetic radiation from a lamp onto a small region on a sample. The same microscope objective collects reflected electromagnetic radiation from the sample which is then directed by suitable beamsplitters and or other optics to a detector. The main drawback to this method is that the practical usable wavelength range is primarily limited to visible and near infrared regions of the electromagnetic radiation spectrum due to the extreme difficult) of designing a refractive objective that simultaneously spans the UV, visible, and near infrared portions of the electromagnetic spectrum.
This difficulty is primarily due to two reasons: a) the extreme difficulty of designing a color corrected objective due to the limited availability of materials that a lens designer has at his/her disposal that transmits in the UV portion of the spectrum and do not exhibit birefringence; and b) the extreme difficulty of designing and producing antireflection coatings for the lens elements of the objective that simultaneously covers the UV, visible, and near infrared portions of the electromagnetic radiation spectrum. U.S. Pat. No. 6,587,282 addresses designing a broadband refractive objective for use between 185 and 900 nm by using a three-element objective. However, this patent does not address the difficulty in designing and producing antireflection coatings that covers the 185-900 nm wavelength range.
Another method is to use an all-reflective type objective with spherical mirrors. Cassegrain, Gregorian, and Schwarzschild arrangements are examples of such objectives. These all-reflective objectives have several advantages over refractive objectives. They are completely achromatic and as such are only limited in wavelength range by the availability of reflective coatings that cover the desired region of the electromagnetic spectrum. Also, aberrations due to spherical mirrors are typically much less than those of equivalent refractive elements.
The major drawback to these types of objectives is that they all have central obscurations in the aperture. This central obscuration greatly reduces system efficiency. One can compensate by using an objective with a high numerical aperture (NA). However, this introduces complexities in the extraction algorithm for the thin film thickness and index of refraction since the measured reflectance must in general be modeled as a weighted integral of the reflectance summed over angle of incidence. This requires that one know the intensity versus angle distribution of the electromagnetic radiation which can be further complicated by the fact that this intensity/angle distribution may have wavelength dependency. Furthermore, by using a high NA, the polarization state of the incident electromagnetic radiation also becomes important and must be known and or controlled.
Another method is to use a catadioptric design that employs a combination of spherical mirrors and refracting elements. The purpose of the refracting elements is to correct the aberrations due to the spherical mirror(s). These arrangements are also difficult to design and produce antireflection coatings for, and also have the above mentioned problems related to central obscuration of the aperture.
Another method is to use all reflective off-axis objectives. These objectives do not possess a central obscuration in the aperture. They may be constructed with combinations of spherical and or aspherical mirrors. Typically, prior art designs employ three mirrors and are very sensitive to alignment.
Another method is to use multiple objectives on a rotating turret or linear actuator, each color corrected for a certain region of the electromagnetic spectrum. This is very time consuming since the each objective must be positioned and focused to the correct height in order to take a measurement. Also, insuring that each objective measure from the same region of the sample becomes quite complicated.
The present invention overcomes the above-discussed limitations of the prior art.
It is often desirable to measure polarized reflectance data at near-normal incidence. One example application where measurement of polarized data is useful is in the measurement of critical dimensions (line width, step height, and sidewall angles) of patterned semiconductor wafers. Critical dimension test patterns typically include sets of parallel lines produced on a wafer. The wafer with the patterned parallel lines is placed in the instrument.
The actual angle that the parallel lines make with respect to established axes of the instrument is, in general, not known. It is highly desirable that the measurement is independent of sample orientation, or in other words, the instrument is able to, as part of the measurement, detect or measure the actual rotational orientation of the fast-axis of the sample.
In the following discussion, the source path is the path the electromagnetic radiation takes in traveling from the source of the electromagnetic radiation up to and before reflection from the sample. Also, in the following discussion, the detector path is the path the electromagnetic radiation takes after reflecting from the sample and traveling to the detector.
By inserting a rotatable polarizer that is in both the source (forward) path and detector (return) path of the electromagnetic radiation incident upon and emergent from the sample, a normal incidence reflection ellipsometer is achieved. This type of ellipsometer, where a single polarizing element acts as both polarizer of the incident electromagnetic radiation and analyzer of the reflected electromagnetic radiation from the sample, is capable of measuring ellipsometric parameters psi and delta as well as the sample's orientation of the fast axis with respect to previously established axes of the instrument.
At some point in the path of the normal incidence ellipsometer, due to the facts that the detector and illumination source cannot physically occupy the same volume and that the source and detector paths are nearly coincident at the sample, the source illumination path must be separated from the detector path. This requirement has been handled in several different ways by the prior art.
In general, the prior art falls into three different categories, as discussed below.
(1) Separation of source and detector paths is accomplished via a polarizing beamsplitter. In this arrangement, electromagnetic radiation from a source is first transmitted or reflected by a polarizing beamsplitter. It then impinges on a sample, is reflected by the sample, is reflected or transmitted by the polarizing beamsplitter, and is then transmitted by a rotatable analyzer towards a detector. This arrangement has a significant drawback in that the sample must be rotated in order to determine the orientation of the fast axis of the sample. Also, this arrangement does not allow for measurement of the full possible range of the ellipsometric parameter, delta. Delta is limited to 0 to 180 degrees, instead of 0 to 360 degrees.
(2) Separation of source and detector paths is accomplished by designing a system with a non-zero angle of incidence (near normal angle of incidence) at the sample. In this arrangement, the detector and source paths are never coincident. Examples of this type of ellipsometer are described in Kamiya et al, Phys. Rev. B 46, 15894 (1992c) and Aspnes et al, J. Vac. Sci. Technol. A 6, 1327 (1988b). Due to the angle separation and displacement of the beams, these systems typically must have separate polarizer elements to perform the polarizing and analyzing functions. Having two polarizers instead of a single polarizing element is more expensive and adds complexity to the ellipsometer calibration and sample measurement.
(3) Separation of source and detector paths is accomplished via a non-polarizing beamsplitter. In this arrangement, electromagnetic radiation from the source is first transmitted or reflected by a non-polarizing beamsplitter; then transmitted by a rotatable polarizer, impinges on the sample, it is then reflected by the sample, is transmitted by the rotatable polarizer, and is reflected or transmitted by the non-polarizing beamsplitter towards a detector. These systems have the advantages of a single polarizing element, and that ellipsometric parameters, psi and delta, and the relative orientation of the fast axis of the sample with respect to previously established axes of the system, are directly measured.
One significant drawback to this system is that it is very difficult to design and produce a 45 degree (45 degrees is desirable for an easy to align compact system) broadband non-polarizing beamsplitter that effectively covers the UV, visible and near infrared regions of the electromagnetic spectrum. Also, calibrating the system (ellipsometer) to account for the necessary correction parameters due to such a non-polarizing beam-splitter adds significant complexity to the ellipsometer calibration. If the non-polarizing beamsplitter is perfect, no correction parameters are needed. A perfect non-polarizing beamsplitter reflects incident s and p polarized electromagnetic radiation equally, and transmits incident s and p polarized light equally as well.
If one arranges the system components so that the angle of incidence at the non-polarizing beamsplitter is very small, then the design of the non-polarizing beamsplitter becomes much more feasible. An example of such an arrangement is given in Cui et al, Applied Optics, Vol. 35, No. 13, 2235-2238, 1996. In this arrangement, the angle of incidence at the non-polarizing beamsplitter is less than 1 degree.
One significant drawback to this type of system arrangement is that the detector path after reflection from the non-polarizing beamsplitter travels back towards the sample almost coincident and in the same direction as the source path. In order to prevent the detector from blocking the electromagnetic radiation from the source reaching the sample, this requires that the distance between the non-polarizing beamsplitter and the sample be quite long; in other words, this type of arrangement does not lend itself to a compact system design.
Another problem associated with the general arrangement of a single polarizing element common to the source and detector paths), is that electromagnetic radiation reflected from the polarizer itself may reach the detector. This reflection can normally be subtracted from the measurement by performing a suitable background measurement. Nevertheless, it is highly undesirable since it effectively degrades the system signal to noise ratio and makes measurement of samples with very low reflection highly problematic.
Typically this reflection from the polarizer that reaches the detector is limited by applying antireflection coatings to both faces of the polarizing element. For broadband ellipsometers, this is problematic because as mentioned previously, it is extremely difficult to design and produce effective antireflection coatings that simultaneously cover the UV, visible, and near infrared portions of the electromagnetic radiation spectrum.
The present invention also overcomes these limitations of the prior art.